Modified Stirling cycle cryogenic refrigerators of the type described in the U.S. Pat. No. 3,074,244 have proved to have substantial advantages over other known types of refrigeration systems. Such refrigerators are inherently lighter, less expensive, more reliable and more efficient than any other available system. Additional important advantages are that they operate using non hazardous working gases, such as helium or nitrogen and require no condenser or evaporator coils.
Such modified Stirling cycle refrigerators are frequently driven by an electrical motor. A problem that has been experienced particularly with large size refrigerators of this type is that when the refrigerator is turned off by removing power from the electrical motor, the temperatures difference between the cold head and the body of the refrigerator can cause the refrigerator to run spontaneously in reverse as an engine, thus tending to warm the cold head. This resulted in substantial reductions in the refrigerator's efficiency since it was essentially working against itself.
A further problem existed in that the starting torque of the refrigerator caused the electrical motor to experience a large starting current surge and the motor and winding had to be large enough to support it. This increased the cost of the system.
To overcome those problems a bypass channel and a solenoid controlled bypass valve, also referred to as an unloader valve, have been incorporated into some refrigerators. When the unloader valve is open, the channel bypasses the compressor piston so that the pressure build-up in the compression cylinder during the compression cycle is minimized. In this system the unloader valve is actuated to the closed position by the solenoid after a time delay each time the electric motor is turned on during operation of the refrigerator, and is held closed by the solenoid for as long as the refrigerator operated. The unloader valve is opened when the electric motor is turned off. The bypassing of the compressor piston by opening the unloader valve both reduced the starting torque and prevented the refrigerator from running in reverse when the power was removed.
A problem still existed however in that the solenoid had to be operated continuously while the refrigerator was operating, which caused a heat buildup within the refrigerator that had to be dissipated and which decreased the efficiency of the system.